1. Field of the Invention
The invention pertains to the field of mechanical power transmission chains for continuously variable transmissions. More particularly, the invention pertains to a CVT chain-belt of the link-and-pin type.
2. Description of Related Art
In recent years, significant research and development has been devoted to practical continuously variable transmissions (CVT) for automotive applications. A CVT provides a portion of the mechanical link between the vehicle engine and the drive wheels used to control the torque output of the engine.
A CVT generally operates by the use of multiple variable pulleys mounted on parallel axes, connected by an endless chain-belt, typically comprising metal or elastomeric materials. A first variable pulley is situated on an input shaft and is mechanically driven by the vehicle engine. A second variable pulley is mounted on an output shaft and is driven by the first pulley through the chain-belt. The second pulley acts through additional drive components to transmit torque to the vehicle drive wheels. Each pulley rotates about an independent shaft and is formed by the cooperation of two pulley sheaves, one of which is axially movable in a direction opposite from the other. The sheave pairs, mounted on the pulley axis, form the inner faces of the pulley. The profiles of the inner faces are generally inclined, such that the two sheave inner faces tend to converge toward the pulley axis. When at least one of the sheaves is movable axially relative to the other sheave, variation in the distance separating the opposing inner faces can be obtained.
The contact surface of the chain or belt, which serves as a power transmission element, engages the inner faces of the pulley sheaves and transmits torque by friction. Most conventional load block configurations include load blocks having a contact surface which is a plane surface. In such configurations, the contacts with conventional conical sheaves are line contacts (i.e., where the contacted area is a band) and not point contacts. As the chain-belt passes over the pulley, the point where the load block or link articulates is defined as the pitch-line. The pitch (p) is the distance between successive articulations.
During the operation of the CVT, a movable sheave on the first pulley may be translated axially along the pulley axis so as to increase or decrease the distance separating the sheave inner faces of the first pulley. At the same time, a movable sheave on the second pulley decreases or increases the distance separating the sheave inner faces of the second pulley, thus maintaining the total required chain length for the loop around the two pulleys.
As the sheave members are translated along the axis of the pulley, the effective pulley radius is increased or decreased due to the inclined inner face of the sheave. The location of the chain-belt articulation or neutral axis (i.e., the pitch-line) around the intermediate circumference of the pulley inner faces defines the effective radius of the pulley. As the sheave inner face separation distance of the first pulley decreases, the chain-belt is forced to adopt its contact at a larger radial distance as it rises up along the inclined sheave inner face and the pitch-line is changed. Simultaneously, the effective radius of the second pulley is proportionately decreased by the separation of the pulley sheaves therein. Similar to the first pulley, the chain-belt is forced to adopt its contact at a smaller radial distance and the pitch-line is changed. Thus, the ratio of the pulley radii may be varied continuously to obtain the desired final drive ratio for the specific vehicle operating conditions. Through this process the CVT changes the xe2x80x9cgearxe2x80x9d ratio of the transmissionxe2x80x94the ratio of the speeds of the input and output shaftsxe2x80x94in a continuous fashion.
Typically the inclined inner pulley sheave faces are generally linear (conical). However, curved profile inner sheave faces also are used to reduce the overall profile of the CVT pulleys.
In the past, the most common configuration for the chain-belt which mechanically links the pulleys has been a conventional chain-belt having a plurality of interconnected load blocks, and may have a variety of link and block configurations, e.g. pin or rocker chains, link belts, etc. Such a chain transmits power in a conventional way by transmitting a pulling force through the links and pins of the chain. This type of chain has drawbacks in CVT service, because of the very large compression force required to transmit power from the sheaves to the transmission belt. When a conventional chain is used in a CVT, this compressive force causes the chain to wear and can deform the pins and links of the chain.
Various attempts have been made in the prior art to address the problem of CVT chain wear. U.S. Pat. No. 3,431,724 to Steuer discloses a chain having links of unitary construction with wear pads located in the center of the outer surface of the links. U.S. Pat. No. 4,944,715 to Ueda et al. discloses a chain wherein the outer link plates include central wear pads that are integral with the chain link plates. However, one problem with such prior art chains is that, because the wear pads are located on the chain links themselves, the large compression force of the CVT pulleys squeezes the chain links together, thereby generating unwanted friction between facing pairs of link plates, and potentially bending or crushing the links. U.S. Pat. Nos. 5,052,985 and 5,306,214 to Masuda et al. disclose chains having wear pads located over the pins. One problem with these prior art chains is that, because the wear pads are located over the pins, the large compression force of the CVT pulleys squeezes the pins, thereby potentially bending the pins or damaging the corresponding links. In a multi-plate link chain, the compression of the outer plates with the load blocks can squeeze the inner links together, creating increased resistance to the necessary bending of the chain.
A chain-belt for a CVT having a plurality of links, each link comprising a plurality of parallel tensile link plates, each link plate having connection apertures located substantially towards the ends thereof, each link being connected to an adjacent link by a pin having two ends passing through overlapping connection apertures in adjacent link plates. Wear pads are positioned on said outer link plates. A bushing, at least as long as the thickness of the tensile link plates, coaxially surrounds the chain""s pins, passing through oversized holes in the link plates. Compressive force is transmitted between the wear pads by the bushing, so that the compressive force exerted by the sheaves does not deform the pin or squeeze the link plates together. The wear pads are centered on the centerline of the chain, and taper to match the taper of the pulley sheaves. The wear pads are also designed so the centerline of the wear pads intersects the center axis of the pulley when the chain is in use.
For a better understanding of these and other aspects of the invention, reference should be made to the following detailed description taken in conjunction with the accompanying figures.